This invention relates to characterizing mechanical properties of materials.
In general, the field of material mechanics is concerned with stresses and deformations of bodies. More specifically, material mechanics is generally thought of as the science which establishes the relationships between forces applied to a body and the resulting deformation of the body. The three fundamental types of stresses are tension, compression and shear. The manner in which a material responds when subjected to any or all of these stresses can be used to characterize mechanical properties of a material. For example, when a material is placed in tension, its tensile properties (e.g. tensile modulus) can be determined by measuring the deformation of the material as a function of the applied tensile stress.
A wide variety of different materials have been characterized for their mechanical properties using as many different techniques and instruments. For example, the compressive properties of articular cartilage tissue and prosthetic cartilage have been analyzed to assess the relative health of the tissue or material and the viability of the replacement material.
Articular cartilage is the bearing material in synovial joints responsible for transmitting and distributing the relatively large compressive loads arising in joints. It also provides a bearing surface with a very low coefficient of friction. As cartilage first begins to break down (e.g., due to osteoarthrosis (OA)), accurate knowledge of its mechanical properties is of great relevance. Further, knowledge of the mechanical properties is essential in attempting to produce a prosthetic cartilage for repairing surface damage of joints.
Articular cartilage is a complex structure, generally devoid of nerves and avascular, although a few blood vessels may be found in its deepest parts adjacent to the bone. Articular cartilage tissue consists of an abundant extra cellular matrix having a large amount of water (75 percent by wet weight) of which a large proportion is freely exchangeable with the synovial fluid bathing the tissue. When a compressive load is applied to articular cartilage there is an instantaneous deformation due to the elastic properties of the matrix followed by a creep phase which is due to the movement of fluid from the cartilage matrix into the surrounding synovial fluid. When the load is removed, there is an instantaneous recovery followed by a time dependent recovery before articular cartilage returns to its initial state. As the articular cartilage begins to break down, the matrix becomes less resilient. Thus, compressive property measurements can be used to distinguish healthy cartilage from unhealthy cartilage.